1. Field of the Disclosure
This invention pertains to a printing form precursor and a method for preparing a printing form from the precursor, and in particular to a photosensitive printing form precursor having an elastomeric cap layer containing particulate and a method for preparing to form the printing form with a relief surface.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials including corrugated carton boxes, cardboard boxes, continuous web of paper, and continuous web of plastic films. Flexographic printing plates are a form of relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. The multilayer cover element includes a layer of an elastomeric composition, i.e., an overcoat layer, that is adjacent and in contact with the bulk photopolymerizable layer as disclosed by Grutzmacher et al. in U.S. Pat. No. 4,427,749.
Flexographic printing forms are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically, the printing form precursor is uniformly exposed through its backside, i.e., backflashed, to a specified amount of actinic radiation to form a floor, and is imagewise exposed through its front side with the same actinic radiation that was used for the backflash exposure. Imagewise exposure can be through an image-bearing art-work or a phototool, such as a photographic negative or transparency (e.g. silver halide film), that is held in intimate contact under vacuum to the photopolymerizable layer, so called analog workflow. Alternatively, imagewise exposure can be through an in-situ mask having radiation opaque areas and clear areas that had been previously formed above the photopolymerizable layer, so called digital workflow. The precursor is exposed to actinic radiation, such as ultraviolet (UV) radiation, to selectively cure the photopolymerizable layer. The actinic radiation enters the photosensitive element through the transparent (or clear) areas and is blocked from entering the photopolymerizable layer by the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer, which are exposed to the actinic radiation, cure or hardened and crosslink. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the phototool or in-situ mask during exposure do not crosslink or cure (i.e., harden). The uncured regions are soluble to solvents used during washout development and/or can melt, soften, or flow upon heating. The printing plate precursor is then subjected to a developing step wherein the unexposed areas (i.e., uncured areas) are removed by treating with a washout solution or heat leaving a relief surface with an image suitable for printing. If treated with washout solutions, the printing form or plate is subsequently dried to remove solvents that may be absorbed by the plate. The printing plate can be further exposed to UV radiation to ensure complete polymerization and to remove surface tackiness. After all desired processing steps, the plate or printing form is then mounted on printing press to print the formed relief image onto a substrate.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Since in analog workflow there is intimate contact and a vacuum between the precursor and the phototool, imagewise exposure to actinic radiation occurs in the absence of oxygen so that the polymerization of the photopolymerizable material occurs at a rapid rate and generally matches the dimensions of the transparent areas of the mask, i.e., mask openings. Dots produced by analog workflow are typically conical and have a flat-top. However, because the opaque elements of the image-bearing artwork are separated by the negative film from the precursor, typically some loss of resolution occurs. Actinic radiation spreads between the phototool mask elements and the surface of the precursor and thereby limits the resolution that is possible.
An alternative to analog workflow is digital workflow, which does not require the preparation or use of a separate phototool. Photosensitive elements suitable for use as the precursor capable of forming the in-situ mask in digital workflow are described in U.S. Pat. No. 5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; U.S. Pat. No. 6,929,898; U.S. Pat. No. 6,673,509; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,037,102; and U.S. Pat. No. 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to infrared radiation and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. The precursor is overall exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen, i.e., about 21% oxygen, (since no vacuum is needed). Furthermore, due in part to the presence of atmospheric oxygen during main exposure the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow results in the relief image having a different structure of the raised surface areas. In particular, the fine raised surface of dots (i.e., the individual elements of a halftone image) is typically smaller (than the actual mask opening), with a rounded top, and a curved sidewall profile, which is often referred to as dot sharpening effect. The printing form has improved resolution and dot gain since the in-situ mask is integral with the surface of the photopolymerizable layer and avoids actinic radiation spreading between the mask element and the surface of the precursor. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. The smaller fine raised surface of the dots also helps compensate for deformation of the elastomeric printing form during printing. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form.
However with ever-increasing demands on quality, the current state-of-the-art relief printing forms for flexographic printing may not perform as desired. It is desirable for flexographic printing forms to print solid areas with uniform, dense coverage of ink. Poor transfer or laydown of ink from the printing form to the substrate, especially in large areas, results in print defects, such as mottle and graininess. Flexographic printing forms having high durometer, that is printing forms having a Shore A greater than 65, oftentimes exhibit poor ink laydown. Unsatisfactory printing results are especially obtained with solvent-based printing inks, and with UV-curable printing inks.
There are a number of ways to try and improve the ink density in solid areas of an image printed by a flexographic printing form. One way to improve solid ink density is to increase the physical impression between the printing form and the substrate. While this will increase solid ink density, the increased pressure will tend to deform smaller plate elements resulting in increased dot gain and loss of resolution. This is especially apparent when printing on critical substrates like, for example, foils, where a lot of print defects can be observed. Similarly changes in the mounting tape used to hold the flexographic plate to the plate roll can also result in increased solid ink density by modifying the way the pressure is distributed across the plate surface. While the proper mounting tape can help mitigate some of the loss in dot gain with increasing pressure, it is still a compromise between the two competing attributes.
Another method of improving solid ink density involves increasing the surface area of the relief printing form. A relief printing form with a roughened surface can hold more ink than smooth surface. However, the surface roughness should be sufficient to increase ink transfer but not so much as to cause discreet features to directly print as this would result in undesirable artifacts in the final print. Bode et al. in WO 2003/079114 disclose a photosensitive element having a matted layer disposed above the photopolymerizable layer, and the matted layer includes a polymeric binder and a specific matting agent capable of anchored in the surface of the photopolymerizable layer. A printing form prepared from this photosensitive element by analog workflow successfully retains the roughened surface, but some instances there can be some loss of the fine structure of the roughened surface when prepared by conventional digital workflow. Rudolph et al. in U.S. Patent Publication 2004/0234886 disclose a photosensitive element having a matted layer disposed above the photopolymerizable layer, and the matted layer includes a polymeric binder and a specific matting agent capable of forming depressions into a plane of the photopolymerizable layer. In this photosensitive element the matted layer forms depressions that create a roughened surface at an interface between the photopolymerizable layer and the matted layer. A printing form prepared from this photosensitive element by analog workflow successfully retains the roughened surface, but some instances there can be some loss of the fine structure of the roughened surface when prepared by conventional digital workflow. Since in conventional digital workflow the exposure is done in the presence of atmospheric oxygen, a portion of the photopolymerizable layer at the interface may not cure. The features that do remain at the surface are relatively large compared with the smallest features in the digital workflow image. This results in missing or deformed features in the resulting printing form that result in print defects.
It is known to include small amount of particles into the photosensitive element to aid in analog workflow or as fillers. Chen et al. in U.S. Pat. No. 4,369,246 disclose the addition of immiscible polymeric or nonpolymeric organic or inorganic fillers, such as organophilic silicas and silicas, to a photosensitive layer in an elastomeric printing relief element. Cushner et al. in U.S. Pat. No. 5,798,202 and U.S. Pat. No. 5,804,353; Kannurpatti et al. in U.S. Pat. No. 6,737,216; and, Hiller et al. in U.S. Pat. No. 6,935,236 disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The elastomeric layer can be reinforced mechanically by incorporating reinforcing agents, such as finely divided particulate material that includes silica and silicates into the elastomeric layer. Laser engraving is a method by which the relief structure of the printing form is created by laser radiation directly impinging the layer sufficient to remove polymeric material (by decomposition in the form of hot gases, vapors, fumes, or small particles) in depth from the relief layer. A mask is not used on the elastomeric layer for laser engraving since the laser can be controlled to selectively remove the polymeric material.
Another way to increase solid ink density printing capability for a relief printing form is through digital patterning of image areas of the precursor as disclosed by Stolt et al. in US Patent Publication 2010/0143841. Stolt et al. disclose applying a pattern to all image feature areas in halftone data that is used to produce an image mask, which is then used to convert the precursor into a relief printing form. After processing, the printing form carries a relief image that resolves the pattern in the surface of the relief features, and provides solid relief features to maintain or increase printed solid ink densities. The problem with this and similar approaches is that they generally require expensive upgrades to the (laser) devices that image the masks. In addition the process is more complicated in order to produce the effect. Samworth in U.S. Pat. No. 6,492,095 discloses a flexographic printing plate having solid image areas which are covered by a plurality of very small and shallow cells. The cells are created via a screened film halftone negative, an intermediate photomask, or via a top layer on the plate that is used as a mask. However, because the digital workflow is exposed in the presence of oxygen the smaller halftone images have to be relatively large. This can sometimes render them ineffective in certain printing applications or can result in visual artifacts.
So a need arises for relief printing form to meet the increasing demands for print quality to improve the transfer of ink to printed substrate and to print, particularly solid areas, with uniform, dense coverage of ink. There is a need for a method that is simple and relatively quick in preparing the relief printing form from a photosensitive printing form precursor, and can provide the printing form with a relief structure that improves transfer of ink to the substrate, without detrimental impact to dot gain and/or image resolution. It is also desirable for the printing form to have a relief structure capable of printing a full tonal range including printing of fine print elements and highlight dots and thereby providing improved print quality.